In order to form a three-dimensional integrated structure, a first integrated circuit may be produced, comprising an insulating material, for example, silicon dioxide (SiO2), on one face and metallic lines on this same face, for example, metallic lines of the highest level of an interconnection network (BEOL: Back End Of Line). A second integrated circuit having a similar structure may be assembled by direct bonding, or molecular bonding, directly on the face. This assembly corresponds to assembling silicon dioxide on silicon dioxide, assembling conductive material (generally copper) on conductive material (for the metallic lines of two different integrated circuits, which overlap), and assembling silicon dioxide on conductive material (for the metallic lines which do not overlap with other lines). The metallic lines which overlap thus form electrical connections between the two integrated circuits.
Not all the lines arranged in the vicinity of the front faces of the assembled integrated circuits are necessarily in contact with a line of another integrated circuit. It may be, in particular, desirable to separate the neighboring lines belonging to two different integrated circuits to reduce undesired contacts between two of these lines. These undesired contacts arise in the event of an alignment error or if the metallic species of at least one of the two lines diffuse towards the other line to form an electrical contact. This diffusion of metal atoms may occur after an anneal of the three-dimensional integrated structure, or during application of an electrical potential difference between the two lines, for example, during operation of the three-dimensional integrated structure.
For this reason, it may be desirable to estimate the diffusion length of the metallic species included in the metallic lines arranged in the vicinity of the interface between the integrated circuits. It has therefore been proposed to use transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), secondary ion mass spectrometry (SIMS), and liquid phase decomposition coupled to atomic absorption spectroscopy (LPDAAS) techniques.
These techniques have the drawback of being destructive. A three-dimensional integrated structure in which theses techniques have been carried out cannot be completed even if the diffusion length is sufficiently short to allow successful operation of the structure.